Alzheimer's disease (AD) is a neurological disorder resulting in the degeneration and eventual death of neurons in brain centers controlling memory, cognition and behavior. The hallmark of the disease is the formation of insoluble amyloid deposits (senile plaques), the major component of which is the 40-42 amino acid amyloid beta (Aβ) peptide, a proteolytic product of the amyloid precursor protein (APP). These plaques are widely believed to be the major causative agents leading to the degeneration and death of neuronal cells.
The three major known genes associated with inheritance of familial Alzheimer's disease (FAD) in humans are the transmembrane receptor amyloid precursor protein (APP) and the two presenilin (PS1 and PS2) genes. Missense mutations in these genes result in the increased production of the Aβ peptide, underscoring the importance of this peptide in contributing to the disease state. APP is cleaved at two sites, beta and gamma, to release a 40-42 amino acid peptide, Aβ (reviewed in Mills, J. and Reiner, P. B. (1999) J. Neurochem 72: 443-460). Missense mutations in APP near the gamma site (Goate, A. et al., (1991). Nature 349: 704-706.), where the C-terminal end of the peptide is cleaved, result in production of more Aβ 42, by altering the 40/42 ratio (Suzuki, N., et al. (1994). Science 264: 1336-1340). Mutations around the beta site result in more overall production of both forms (Mullan, M., et al. (1992). Nat. Genet. 1: 345-347.); Citron, M. et al. (1995). Neuron 14: 661-670).
The presenilins are multiple pass transmembrane proteins, the functions of which are currently a matter of debate. Missense mutations in presenilins increase the release of the Aβ 42 form (Borchelt, D. R., et al. (1996). Neuron 17: 1005-1013); Citron, M., et al. (1997). Nat. Med. 3: 67-72; Murayama, O. et al. (1999). Neurosci. Lett. 265: 61-63) and account for the majority of FAD cases (Sherrington, R., et al. (1995). Nature 375: 754-760).
Many studies have examined the roles of both the soluble and insoluble (aggregated) forms of Aβ and it is widely believed that the aggregated form of the peptide is responsible for the observed toxic effects (Pike, C. J., et al. (1993). J. Neurosci. 13: 1676-1687; Lorenzo, A. and Yankner, B. A. (1994). Proc. Natl. Acad. Sci. USA 91: 12243-12247; Giovannelli, L., et al (1998). Neurosci. 87: 349-357). There are a number of mechanisms that contribute to Aβ-induced death of neurons, including the disruption of intracellular calcium levels (for reviews, see Fraser, S. P., et al. (1997). Trends Neurosci. 20: 67-72; Mattson, M. P. (1997). Physiol. Rev. 77: 1081-1132; Coughlan, C. M. and Breen, K. C. (2000). Pharmacol. and Ther. 86: 111-144), the induction of an inflammatory response caused by activation of microglial cells (reviewed in Coughlan, C. M. and Breen, K. C. (2000). Pharmacol. and Ther. 86: 111-144) and the marked degeneration and/or disruption of the basal-forebrain cholinergic system, which is involved in learning and memory (reviewed in Hellstrom-Lindahl and Court, 2000, Behav. Brain Res. 113 (1-2): 159-68). Thus, it is clear that the deleterious effects of Aβ overproduction and its contribution to AD are numerous and complex.
Although a great amount of research has been dedicated to the study of Alzheimer's Disease and its general pathology, the genetic analysis of human neurodegenerative disorders is limited. As a result, the events that trigger the accumulation of beta amyloid, as well as the precise role of genes such as APP and others suspected to play a part in Alzheimer's Disease, is poorly understood.
Numerous contributions to the establishment of the central role of Aβ in the manifestation and progression of AD have come from studies in model systems. Transgenic mice expressing either wild type or mutant forms of APP exhibit AD pathology, in many cases developing amyloid plaques in an age-dependent fashion and in some cases displaying altered behavior and cognition (for reviews, see Price, D. L., et al (1998). Annu. Rev. Genet. 32: 461-493; van Leuven, F. (2000). Progress in Neurobiol. 61: 305-312). Transgenic mice expressing only the Aβ 42 peptide exhibit extensive neuronal degeneration in brain regions normally affected in AD, and 50% die at 12 months of age (LaFerla, F. M. et al. (1995). Nature Genet. 9: 21-30). The neural cells in these mice eventually apoptose, followed by astrogliosis and spongiosis. This demonstrates that Aβ 42 expression is toxic in vivo, and results in neuronal degeneration and apoptosis.
The use of Drosophila as a model organism has proven to be an important tool in the elucidation of human neurodegenerative disease pathways (reviewed in Fortini, M and Bonini, N. (2000). Trends Genet. 16: 161-167), as the Drosophila genome contains many relevant human orthologs that are extremely well conserved in function (Rubin, G. M., et al. (2000). Science 287: 2204-15). For example, Drosophila melanogaster carries a gene that is homologous to human APP which is involved in nervous system function. The gene, APP-like (Appl), is approximately 40% identical to the neurogenic isoform (695) of the human APP gene over three large domains (Rosen et al., PNAS USA 86:2478-2482 (1988)) and, like human APP695, is exclusively expressed in the nervous system. Flies deficient for the Appl gene show behavioral defects which can be rescued by the human APP gene, suggesting that the two genes have similar functions in the two organisms (Luo et al., Neuron 9:595-605 (1992)).
In addition, Drosophila models of polyglutamine repeat diseases (Jackson, G. R., et al (1998). Neuron 21: 633-642; Kazemi-Esfarani, P. and Benzer, S. (2000). Science 287: 1837-1840; Fernandez-Funez et al. (2000) Nature 408 (6808):101-6, and Parkinson's disease (Feany, M. B. and Bender, W. W. (2000). Nature 404: 394-398) closely mimic the disease state in humans, both at the cellular as well as the physiological level and have been used successfully to identify other genes that play a role in these diseases. Thus, the power of Drosophila as a model system is demonstrated in the ability to represent the disease state and to perform large scale genetic screens. This invention generally relates to a method to identify compounds and genes acting on the APP pathway in transgenic Drosophia melanogaster ectopically expressing genes related to AD. Expression of these transgenes can induce visible phenotypes and it is contemplated herein that genetic screens disclosed herein may be used to identify genes involved in the APP pathway by the identification of mutations that modify the induced visible phenotypes. The genes affected by these mutations will be called herein “genetic modifiers”. It is contemplated herein that human homologs of genetic modifiers thus identified would be useful targets for development of therapeutics to treat conditions associated with abnormalities in the APP pathway, including, but not limited to, the development of Alzheimer Disease (AD) therapeutics. It is also contemplated herein that some of these human homologs might be occurring on an area of human chromosome 10, shown to be linked to Alzheimer's disease (Bertram et al., Ertekin-Taner et al., Myers et al., Science 290, 2302-2305, 2000). Such human homologs might have the potential to be genetically linked to AD and serve as markers for AD or as targets for the development of therapeutics to treat conditions associated with abnormalities in the APP pathway, including, but not limited to, the development of Alzheimer Disease (AD) therapeutics. Such human homologs might also be acting in cellular pathways involving genes linked to AD and these human homologs might be used to identify the genes in these pathways.